klmb1 mod 11.17 (4)

Issue 1 – 04 Sept 2001 -1

JAR 66 CATEGORY B1MODULE 11.03AEROPLANE

STRUCTURESengineeringuk

Contents

1 AIRFRAME STRUCTURES - AEROPLANES.............................. 1-31.1 FUSELAGE ...................................................................................1-3

1.1.1 Truss Fuselage Construction ........................................1-31.1.2 Truss Fuselage - Warren Truss.....................................1-31.1.3 Stressed Skin Structure.................................................1-41.1.4 Pressurised Structure....................................................1-51.1.5 Attachments ..................................................................1-61.1.6 Passengers and Cargo .................................................1-91.1.7 Doors ............................................................................1-101.1.8 Windows and Windscreens ...........................................1-12

1.2 WINGS.........................................................................................1-141.2.1 Construction..................................................................1-141.2.2 Fuel Storage .................................................................1-161.2.3 Landing Gear ................................................................ 1-181.2.4 Pylons ...........................................................................1-191.2.5 Control Surface and High Lift/Drag Attachments ...........1-20

1.3 STABILISERS ................................................................................1-211.4 FLIGHT CONTROL SURFACES ........................................................1-221.5 NACELLES AND PYLONS ................................................................ 1-23

Page Issue 1 – 04 Sept 20011-2

JAR 66 CATEGORY B1MODULE 11.03

AIRFRAMESTRUCTURESengineering

uk

Issue 1 – 04 Sept 2001 Page 1-3



1 AIRFRAME STRUCTURES - AEROPLANES

1.1 FUSELAGE

The fuselage of a light aircraft is the body of the aircraft, to which the wings, tail,landing gear and engines may be attached. Larger aircraft can have their mainlanding gear attached to the wings and, on multiple engined aircraft, a number ofthe power-plants can be wing mounted also.The loads produced either on the ground or in flight, will at some time, have topass through the fuselage. In order to absorb these tremendous loads imposedupon the structure, the fuselage must have maximum strength, but this must becombined with the other constraint, that of minimum weight.There are two types of construction found in the majority of modern aircraftfuselage design, the truss and the stressed skin type.

1.1.1 TRUSS FUSELAGE CONSTRUCTION

By definition, a truss is a form of construction in which a number of members (orstruts), are joined to form a rigid structure normally covered with non-loadcarrying material such as cloth, fabric or thin sheets of wood.Very early aircraft used a method of construction referred to as a Pratt Truss,where struts were held in compression, and wires, which ran diagonally betweenthe struts, were in tension.

Truss Fuselage – The Pratt TrussFigure 1

1.1.2 TRUSS FUSELAGE - WARREN TRUSS

When fuselages were subsequently made from welded tubes, the Warren Trussbecame popular. In this arrangement, shown overleaf, the longerons areseparated by diagonal members which carry both compressive and tensile loads.




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Warren TrussFigure 2

1.1.3 STRESSED SKIN STRUCTURE

The neccessity of having to build a non-load-carrying covering over a structuraltruss led to designers to develop the stressed skin form of construction. In thismethod, a proportion of the load is carried by the outside skin, which can be alsobe formed into a much smoother and more efficient shape.The commonest form of a stressed skin structure is a chicken egg (puremonocoque). The seemingly fragile shell can resist high loads, as long as theyare applied in a proper direction.

Pure-Monocoque StructureThis form of stressed skin construction is rarely seen in its purest form, because itis normal to add some form of light internal structure to help support the skin.However, there are some aircraft (normally gliders and sailplanes) made fromglass reinforced plastic (GRP), which are constructed as a pure monocoquestructure. In this design, the GRP skin is quite thick, often with a core of someother lightweight material such as balsa wood or composite honeycomb, so thereis no need for any internal, supporting structure.

Semi-Monocoque StructureThis form of construction has a skin carrying a large amount of the loads, but withan internal structure of frames and stringers to keep the skin to its correct shape,where it can best carry the loads. Some have longerons which are moresubstantial than stringers and carry most of the longitudinal structural loads, withthe frames carrying the radial loads.




1.1.4 PRESSURISED STRUCTURE

High altitude flight places the occupants in a hostile environment in which lifecannot be sustained without oxygen. To avoid the need to wear oxygen masks,the pressure in the cabin is raised higher than it is outside, which providessufficient oxygen in the air for the passengers to breathe normally.In the 1950’s, piston-engined aircraft, had a pressure differential across the cabinwall about two pounds per square inch (psi) maximum. Modern aircraft cabinscan sustain a pressure differential between 8 and 10 psi, so there must not beany part of the structure containing 'stress raisers' which would concentratestress to an unacceptable level. Much of the structure of modern aircraft hasbeen built to the 'fail safe' philosophy, in which the structure is built with multipleload paths for the major stresses to pass through, to cater for the unlikely failureof a single structural item.

Pressurisation SealingAll joints in the structure, as well as openings such as doors, panels, emergencyexits, etc. must be completely airtight during flight, to prevent the cabin pressureleaking below its required level. Joints are constructed with an interface of sealingcompound, whereas windows and doors employ pre-formed rubber seals aroundtheir edges. The points where control tubes and cables pass in and out of thepressure hull, utilise some form of flexible bellows which are leak proof but movewith the controls.

Pressurisation SealingFigure 3




uk1.1.5 ATTACHMENTS

The fuselage can, as mentioned earlier, carry most of the major loads, both onthe ground and in flight. To this end, most of the other airframe components suchas the wing, stabilisers, pylon and undercarriage, can be fitted to the fuselage.The wings can be mounted above or below the passenger compartment. Asalready mentioned, wings are usually attached to the fuselage with multipleattachments, although light aircraft may still have wings attached with as few astwo bolts.

Early High Stress AttachmentFigure 4

Multiple Fastener Wing AttachmentFigure 5




The horizontal and vertical stabilisers can be fitted to the fuselage in numerousdifferent ways. When the horizontal stabiliser is fitted part-way up or on the top ofthe vertical stabiliser, there will be only one strong attachment point. Otherwise,there will be separate attachments for the fin and for the left and right tailplanesections.Where a moving horizontal stabiliser is employed, the attachment will consist ofleft and right rear pivot fittings and a single forward attachment to a trim actuator.On rare occasions, the rear fuselage is manufactured, together with thestabilisers, as one integral unit. Because the loads generated by the empennage,it is usual to find that the rear fuselage structure has stronger frames around thestabiliser attachment points. These frames transmit the loads along the fuselageand away from the tail.The same technique is used when the engines are attached to wing or to rearfuselage mounted pylons The Fokker 70/100, for example, has oblique frames toconnect the vertical stabiliser to the top mounted tailplane and to the fuselage,plus two heavy frames to transmit all the engine thrust loads into the fuselage.

Strengthened FramesFigure 6




ukAs previously mentioned, the landing gear can be attached either to the fuselage,the wings, or within wing mounted engine nacelles. Because of the need for cabinspace, fuselage mounted landing gear on passenger and freight-carrying aircraft,often have the main landing gears mounted in fairings or nacelles beneath thefuselage as in the ATR-72, detailed below.

Faired ATR 72 landing GearFigure 7

The landing gear, as for the other attachments, is mounted on to strong fuselageframes which in this case, are also used to mount the wings, attached above thefuselage. The loads that these frames carry, both in flight and on the ground, aretransmitted into the fuselage by means of longitudinal stringers and longerons.

Fuselage Strong PointsFigure 8




1.1.6 PASSENGERS AND CARGO

Aircraft that carry passengers as well as crew, all have to have seats that complywith crashworthiness regulations. These regulations dictate that the seats with aperson correctly strapped in place, must be able to survive a sudden stop of over20 times the force of gravity, (20g), without the floor mountings (to which the seatis attached) failing, or the seat itself collapsing.Although aircraft seats appear to resemble normal domestic seats, the tubularframework and floor attachment 'feet' are very strong, yet are light in weight andcan be disconnected from the floor if necessary, by releasing a few quick-releasefasteners.Passenger compartment floors of modern aircraft are often panels of thecomposite material ‘Fibrelam’, which are strong enough to carry most of thegeneral loads created by passengers and galley equipment. The panels arethemselves supported by lateral and longitudinal beams, which are primarystructure, into which the panels fit. Lateral beams are attached to the lowerportion of the (usually) circular fuselage frames and longitudinal beams supportedby the lateral beams, are those upon which the seats are fitted.

Seat Track FittingsFigure 9

The top of each longitudinal beam is fitted with location holes which are astandard size and into which all seats are slotted. Additionally, the galleys andbulkhead partitions can also be attached to them. The frequent and equal spacingof the seat track attachment holes, allows the seats to be fitted at a variableincrement, or pitch, to cater for different classes of cabin (economy or first class).On some aircraft, such as the Fokker 100, there are five longitudinal seat tracksin the cabin floor which allow a five abreast seating to be installed (3+2 or 2+3),with the off-set aisle on whichever side the customer wishes.




ukCargo Loading SystemsAircraft which are used for carrying all or part freight loads have to have the floormodified to allow the movement of pallets or containers.Usually this will consist of substantial reinforcement of the flooring with tracks,guides and rollers fitted, to allow safe and easy motorised movement up anddown the freight bay. In the entrance door area, a ‘ball-mat’ is installed to allowthe freight to be easily loaded, rotated and man-handled on to the rollers.

1.1.7 DOORS

This topic covers most methods of entry and exit from the fuselage, includingthose for passengers, crew, refreshments and meals, baggage and majormaintenance access. In addition, some doors are dedicated to emergencies onlyand will therefore remain unused during normal operations.If the aircraft has a cabin pressurisation system, the doors have to be moresubstantial than for a non-pressurised type and be fitted with safety devices toprevent accidental opening. One method to prevent this happening is allow thedoor to open inwards so that the door 'plugs' the aperture when closed and isheld in place by the cabin pressure in addition to the door frame locating bolts.Any door on pressurised aircraft that does open outwards, must have additionaldevices and protection mechanisms fitted to prevent accidental opening and aflight deck warning system to inform the crew if it is not properly closed andsecured.Non-pressurised aircraft doors still have to be safe, with a system of handles andlatches that operate in a specific order or after the application of a certain force.Doors on most aircraft are constructed in a similar way to the fuselage with aninner and an outer skin and vertical and horizontal members. The sometimescomplex locking and latching mechanisms, plus the indicating and warningelectrical wiring systems are all contained within this structure.Most fuselage doors are operated manually, but much larger freight/cargo doorsare either electrically or hydraulically operated. Another requirement on all cabindoors, (normal exit/entry and emergency type) is the need for efficient emergencyegress in the event of a mishap on the ground. They must be operable by asingle handle whose operation shall be ‘rapid and obvious’. Most doors havedecals and large red arrows, to clearly indicate the way in which the handles areto be rotated or moved to open the door.Dedicated emergency exits are almost always 'plug' type and, therefore, cannotbe opened in flight due to the cabin pressure acting on door opening mechanism(usually an over-centre type a cam arrangement) thus preventing handle rotation.




Door MechanismFigure 10

Door Structure and SealingFigure 11




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To prevent leakage of the cabin pressure, all doors have to have a substantialseal around their edges to keep the aperture between door and surroundingfuselage frame airtight. Some seals just compress and fill the space when thedoor is closed, others use cabin air to inflate and therefore expand the seal toachieve the same result. Fig 11 shows a typical door seal arrangement.

1.1.8 WINDOWS AND WINDSCREENS

All the transparencies on non-pressurised aircraft are normally made from acrylicor some other clear plastic material. On pressurised aircraft, flight deckwindscreens have to comply with very strict bird-strike regulations and are madefrom a toughened sandwich of glass/plastic/glass The passenger cabin windowsare manufactured from acrylic, mylar or other plastics.It must be considered that an aircraft travelling at 400 knots which collides with abird weighing 3kg, could suffer severe structural damage, engine failure and moreimportantly, if the bird struck a windscreen and broke through, it could causeserious injury. Furthermore, rapid decompression of the pressure cabin wouldresult. The regulations state that during testing, when a dead bird is fired at itfrom a large air gun, the screen must be able to survive the impact.Consequently, the glass/plastic/glass sandwich is fitted with a heating elementbetween the interface of the front glass panel and the plastic core. Not only doesthe heater provide anti-icing protection, but helps absorb impact since it makesthe plastic core more pliable and shock absorbent. The section through a typicalwindscreen below shows how the lamination of glass and plastic layers isarranged.

Windscreen ConstructionFigure 12




Passenger cabin windows are almost always made from acrylic plastic. Thissaves quite a lot of weight as well as cost. For added safety, the acrylic cabinwindows are actually two layers with a space in between, so that if one fails theother will carry the pressurisation loads, a typical case of fail safe. In addition,some cabin window assemblies have a third, pane of acrylic fitted to help reducethe engine noise in the cabin from the power-plants outside.

Passenger Cabin WindowFigure 13

Most aircraft require one or more flight deck windows that can be opened forsignalling to the ground-crew, for fresh air ventilation if the air conditioning is 'off'on the ground and to be able to see out in emergency situations, for example, thewindscreen becoming obliterated. To achieve this, aircraft are usually fitted with apair of opening front corner or side windows, sometimes called Direct Visionwindows. If the cabin is pressurised, they will be unable to be opened due to theprovision of a similar ‘pressure on’ safety lock system as the cabin doors.




uk1.2 WINGS

1.2.1 CONSTRUCTION

The methods by which the wings produce lift were covered in Module 8, so thismodule will concentrate on wing construction and their attachments.To classify the many types of wing it is best to break them down into differentgroups. The first sub-division is either those that are externally braced or thosethat are of cantilever construction. (no external bracing). In the early days themajority of aircraft were constructed with the whole aircraft, including the wings,being braced by wires and struts. These produced very high drag, although theoverall structural weight could be kept down.As materials and the wing construction became stronger, the number of wires

were progressively reduced, until in the mid-1930's the first genuine fullycantilever wings with no external bracing, were put into production. This does notmean the bracing has been eliminated, it just means that all ‘bracing’ is includedwithin the wing structure and made much stronger. Fig 14 below, shows how theexternal bracing of a biplane has been replaced with more efficient internalbracing on a cantilever wing.

Biplane and Cantilever Wing BracingFigure 14

To illustrate how complex the inside of even a small aircraft wing can be, thefollowing two pictures show the internal structure of both a wood and a metalwing.

Internal Wing Structures – Wood and MetalFigure 15




The heart of a wing is the spar (or spars), to which are attached the ribs stringersand other structural items. The number of spars is decided by the designer ordesign team, but modern airliners normally have two. It is usual to attach landinggears, primary flying controls, leading and trailing edge devices, to one or other ofthe spars within the wing on larger aircraft.Simpler wings on, for example, a light aircraft, will have only one main spar butsome aircraft can have up to five, which has a measure of 'fail safe' philosophy. Ifmilitary aircraft are considered, some modern fighters can have more than 15spars as part of the ‘damage tolerant’ design application.Wing planforms can show an infinite number of different shapes, that are purposebuilt and satisfactory for providing lift. These could be generally grouped intostraight, swept, delta and combination wings. Straight wings include those with aslightly swept leading edge, trailing edge or both.Swept wings are usually categorised as those with both leading and trailingedges swept back, at a variety of different angles, whilst the delta-winged shape(from the Greek for triangle) is self-explanatory.Under the cover-all title of 'Combination', the selection of silhouettes below shouldgive an idea of the wide range of wings that can be found on modern day aircraft,in addition to the more conventional planforms mentioned above.

Wing PlanformsFigure16




uk1.2.2 FUEL STORAGE

Rigid TanksBecause of their shape, wings are often designed to be used for fuel storage.They can either contain separate fuel tanks within the wing structure, or use thewing structure itself, suitably sealed, to make integral tanks.Separate internal tanks are usually manufactured from either light alloy or fromflexible, rubberised fabric. Rigid light alloy tanks are first riveted, then welded tomake them fuel tight and are securely clamped into the wing structure by strapsor tie bars. They will often have baffles inside, to prevent fuel surge from one endof the tank to the other.

Rigid Fuel tankFigure 17

Flexible Fuel tankFigure 18




Flexible TanksFlexible tanks, (Fig 18), also referred to as 'bladder' tanks, have to be locatedsnugly into the tank bay within the wing, because the sides of the bay providesupport to the relatively weak tank skin. Older types of flexible tanks were madefrom rubber- covered fabric. These days the fabric is replaced by man-madefibres, impregnated with neoprene or some similar fuel tight material.

Integral TanksIntegral fuel tanks are found on most, if not all, modern commercial aircraft.During manufacture, practically the entire wing structure becomes a box,comprising front and rear spars, top and bottom wing skins, inboard and outboardsealed ribs, into which are installed pumps, drains, filler caps and vents.The main advantage of the integral tank, is that it provides maximum fuel capacityfor the minimum amount of weight and the only sealing required, is that applied tothe seams after construction is completed.

Boeing 737 Integral Fuel Tank capacitiesFigure19




uk1.2.3 LANDING GEAR

As mentioned earlier, the attachments for major components can often be strongpoints on the wing spars, or even a separate spar built specifically for thatpurpose.. One such component that falls into this category is the main landinggear, otherwise known as the undercarriage. On some very large aircraft, like theBoeing 747 or Airbus A340, additional body gears, as well as conventional winggears are to be found. These have to have reinforcements built into the lowerfuselage structure to absorb the extreme loads at touch down.

Landing Gear AttachmentsFigure 20




1.2.4 PYLONS

Many aircraft have engines mounted on pylons attached to the wing. With this socalled ‘podded engine’ configuration, the pylons have to take very large thrustforces from the engines and transfer it to the airframe. This is normally achievedby attaching the engine to strong points on the pylon and attaching the pylon tothe wing spars. Thrust links are then fixed to the engine frame and the wing sparsto transfer the engine thrust efficiently. Pylons must be positioned low enough sothat the engine exhaust doesn’t strike the wing structure, but not too close to theground to risk a runway scrape. The Boeing 737-600 is a fine example of thiscompromise.

Pylon Engine mountingFigure 21

Turbo-Propeller MountingFigure 22




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Boeing 737-600 Engine Pylon MountingsFigure 23

1.2.5 CONTROL SURFACE AND HIGH LIFT/DRAG ATTACHMENTS

All of the flying controls on the wing will be attached to strong points on either thefront or rear spars. This includes high and low speed ailerons, leading and trailingedge flaps, slats, roll spoilers, speed brakes and lift dumpers. The wing structuremust therefore be made strong enough not only to carry the lift forces in flight butthe additional loads of pilot control inputs, additional drag devices, etc.Consequently, the spars, are always the strongest part of the wing structure.

Control Surface mountings - WingsFigure24

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1.3 STABILISERS

The vertical stabiliser (fin) produces directional or lateral stability, whilst thehorizontal stabiliser (tailplane) produces longitudinal stability. As was mentionedin the aerodynamics section, these surfaces are of similar construction to thewings with spars, ribs, stringers etc,. They have to resist the twisting forces fromthe control surfaces mounted on the trailing edges. In many cases, the fin issimilar to one half of the tailplane and on a number of light aircraft, it is actuallyconstructed in this way, thereby simplifying production and component parts.Light aircraft have stabilisers manufactured from welded tube or fabricated fromthin aluminium sheet of simple construction. As the aircraft size and weightincreases, the surfaces will be made from stronger milled or machined skins andforged spars. Below can be seen examples of the empennage of light aircraft,Piper Cub and Cherokee and Cessna 150, showing their simple construction.

Empennage ConstructionFigure 25




uk1.4 FLIGHT CONTROL SURFACES

The construction of most flight control surfaces is critical, since the designerwants to make them as light as possible. The control surfaces in the early yearsof aviation were a light, tubular frame covered with fabric and in later years whenlight alloy was adopted the quest for lightness continued. Today, metallicstructures with honeycomb cores or epoxy reinforced composite construction areutilised for most control surfaces. The control surfaces are attached to the wing,fin or stabiliser by hinges, the spars being reinforced where these attachmentsare located.The cutaway below shows an elevator from a Fokker 100 and it can be seen thatthe construction is very similar to other main surfaces. The only difference is thatthe rear half of the surface has no internal framework but instead, a core ofshaped aluminium honeycomb with the skin adhesively-bonded to it.

Elevator StructureFigure 26

To prevent the risk of flutter, as previously described, the ailerons, elevator andrudder, are all constructed so that the part of the surface behind the hinge line, isas light as possible and a number of calibrated weights are added to the leadingedge of the surface. These weights are known as mass balance weights, (seecutaway above) and the procedure is known as mass balancing.In addition to mass balancing, surfaces that do not have the benefit of hydraulicpower assistance, (see later) and are difficult to move when the aircraft is at highspeed, have the benefit of aerodynamic balancing. To achieve this simply and aspreviously discussed, the hinge of the control is inset, so that part of the surfacein front of the hinge line projects into the airstream, when the control is deflectedfrom neutral.




1.5 NACELLES AND PYLONS

It has been mentioned previously, how the nacelles and pylons are attached tothe wings, generally and to other parts of the airframe on selected aircraft. Themain purpose of all these engine fairings is to keep the engines outside of theairframe itself. There are several reasons for this, but the major reasons are thatit is safer, in the event of a fire or explosion, if it isolated from the fuselage or thewings by firewalls. Also, it is much easier for routine maintenance and enginechanges, if the engine is externally mounted.Most nacelles are simply fairings which cover the power-plant in a streamlinedmanner, although, they usually also serve as the intake for jet and turbo-propellerengines. Most are covered by large, easy-to-open doors and panels, which allowquick and easy access. On some designs there can be smaller, quick releasepanels fitted into the larger ones, which allow access for maintenance, such as oillevel quantity indicators, which need to be checked every time the engines areshut down.On light aircraft, engine nacelles are usually fairly simple GRP fairings which aresplit into two parts and removed by releasing a few screws or quick releasefasteners. These also contain a small intake for the air to reach the carburettor ofthe piston engine.On many larger aircraft, particularly those with fan bypass engines, are fitted withthrust reversers as part of the cowlings. These are usually doors which translaterearwards and open up panels containing cascade vanes, which re-direct theexhaust thrust in a forward direction, when reverse thrust is selected afterlanding. These will be covered later in the power-plants chapter.Although they are much more efficient that the older designs, modern jet enginesproduce harmful high frequency noise. One way that the noise may be kept belowthe safe and legal minimum, is by making the cowlings out of honeycombsandwich, which as well as being very light in weight is excellent at absorbingsound. The honeycomb can be manufactured from glass or carbon fibre andcovered with composite or light alloy skin facing panels.The pylons which support the engines fitted on to the wings or the rear fuselageall have one main purpose, which is to transmit the full thrust of the engines intothe airframe. They must be extremely strong and yet flexible, as the wing mountsespecially have to move with the flexing of the wings.On many large aircraft, the space within the pylons is utilised to fit suchcomponents as heat exchangers, (radiators); air valves; fuel valves; pipescontaining air, oil and fuel and electric cabling.All engines must be isolated from the rest of the aircraft, so that a fire can becompletely contained within the nacelle and extinguished if the aircraft isequipped a fire extinguishing system. To this end, there will be a sealed bulkheador divider between the engine and the airframe made of a fire resistant materialsuch as titanium or stainless steel.




ukAll engines are subject to vibration that can be sensed inside the aircraft. Toreduce this, the engine mounts are designed not only to hold the engine securelyand to transmit the thrust, but the mounts themselves are fabricated with a shockabsorbing material. This is usually an elastomeric or metallic woven block and willabsorb a large proportion of the vibration providing the passengers and crew witha smooth flight.

Typical Fan Engine CowlingsFigure 27




Cowling and Pylon Fairing InstallationFigure 28




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INTENTIONALLY BLANK

klmb1 mod 11.17 (4)

Documents

fuselagethe fuselage

form of construction

category b1module

early aircraft

light aircraft

larger aircraft

types of construction

method of construction